Generally, a joint is a configuration by which two or more parts are joined. Many kinds of joints exist, such as fixed joints, rotational joints and translational joints. A fixed joint locks two parts together so that they cannot move with respect to each other. A rotational joint, on the other hand, allows the rotation of one part with respect to another part about a common axis. A translational joint allows one part to translate along a vector relative to another part.
Joint structures are classified into various categories depending on the field of study. For example, in anatomy, joints are classified as fibrous, cartilaginous and synovial. These categories classify anatomical joints based upon the magnitude of motion the joint permits. Fibrous joints allow no movement, cartilaginous joints allow a small magnitude of movement, and synovial joints allow the largest magnitude of movement. In robotics, on the other hand, joints are classified as revolute, prismatic, helical, cylindrical, spherical and planar. These categories classify robotic joints based upon the types of motion the joint permits. Joints may also be classified based upon the degrees of freedom of motion the joint permits.
In mechanical engineering, degrees of freedom (DOF) describe flexibility of motion. A mechanism that has complete freedom of motion, even if only in a limited space, has six degrees of freedom. Three modes are translational, each moving in one of three spacial dimensions (e.g. x, y, and z). Three modes are rotational, each changing angles around one of three perpendicular axes. The six degrees of freedom are often described as moving up and down (or heaving), moving left and right (or swaying), moving forward and backwards (or surging), tilting up and down (or pitching), turning left and right (or yawing), and tilting side to side (or rolling).
A single joint structure may provide zero degrees of freedom (e.g. fixed or fibrous joints), one of the six degrees of freedom (e.g. rotational or translational joints) or a combination of the six degrees of freedom delineated above (e.g. synovial joints). Additionally, a combination of joint structures may provide complete freedom of motion.